1. Field of the Invention
The present invention relates to the method of forming joints between copper members, one of which is stranded, which are utilized in transformer parts. More particularly, the invention is directed to the method of forming an assembly containing a solid terminal which is joined to a flexible or stranded copper wire, the joint being made by resistance brazing employing a joining compound of the copper-silver-phosphorus type.
2. Description of the Prior Art
High volume brazing of stranded copper wire to a solid copper part employing phosphorus-silver-copper (hereinafter called "phos-silver") as the joining material can be a real problem when the same is performed on a resistance welder. One of the major difficulties encountered is the fact that the copper parts start sticking to the electrode tips and eventually destroy the usefullness of the tips of the electrodes of the resistance welder.
In high production runs of producing such assembled parts, the individual components or subassemblies are usually loaded on an indexable table in which a solid copper member of the desired configuration is placed in position on which a thin strip or sheet of the joining composition is placed on the solid copper member. Thereafter the stranded copper wire or subassembly is placed in position over the joining material. As thus assembled the electrodes of a resistance welder are placed on opposite sides of the assembly, one of the electrodes being in contact with the solid copper member and the other in contact with the stranded copper wire component. Pressure is applied and the welder is thereafter energized at which time it passes approximately 10,000 amperes through the electrodes and the parts assembled for a duration of about 1 second. During this time, the electrode tips as well as the parts heat up until a sufficient temperature is achieved at which the phos-silver joining composition melts.
Heretofore the tips of the resistance welding electrodes were formed of molybdenum or molybdenum base alloys because at the temperatures employed and with the phos-silver joining material having low wetability for the molybdenum, it was believed that no problem would be incurred of the finished parts sticking to the electrode. However, with the proximity of the stranded wires and the pressures employed acting on the molten material, it was found that the material "wicked" through the wire and came into contact with the electrode contacting the stranded wire. This was apparently caused by the fact that the stranded wire heated up to a higher temperature and more quickly than the solid copper member and as a result, the stranded copper part appeared to act more as a resistor than a conductor. Consequently, after a short period of use the supposed low wetability of the molybdenum tip of the resistance welding electrode was no longer effective and the stranded copper portion of the assembly began to stick to the electrode tip, with the result that the entire operation had to be interrupted and the resistance welding tip replaced with a consequent cleaning of the adjacent parts. This malfunctioning severely limited the number of parts that could be produced within a given time period. In accordance with this invention, it was discovered that a solution to this problem required a heat balance be achieved by minimizing the thermal gradient between the electrode in contact with the solid copper member and the electrode in contact with the stranded copper member, so that both the solid and stranded copper members were at nearly the same temperature at the time the joining material melted.